Electric discharge machining apparatuses perform a machining process on a workpiece by generating arc discharges between a machining-purpose electrode and the workpiece. Electric discharge machining apparatuses require an electric power source (i.e., a machining-purpose electric power supply) that is used for generating the arc discharges. Conventionally, various configurations of machining-purpose electric power supplies are known.
For example, a known electric power supply device for electric discharge machining processes is configured so as to include two power supply units that are namely a finishing-purpose power supply unit and a rough-machining-purpose power supply unit. The device is further configured such that a resistor is connected in series to each of four switching elements structuring a full-bridge circuit included in the finishing-purpose power supply unit and that the resistance values of these resistors are different from one another (see, for example, Patent Document 1).
According to Patent Document 1, a normal polarity machining process, during which a positive potential is applied to a workpiece and a negative potential is applied to a machining-purpose electrode, and a reversed polarity machining process, during which a negative potential is applied to the workpiece and a positive potential is applied to the machining-purpose electrode, are performed. The resistance values described above are set in such a manner that the value of a machining electric current supplied to an electric discharge machining gap is small during the normal polarity machining process and is large during the reversed polarity machining process. With this arrangement, during the normal polarity machining process, a small machining voltage is applied to the electric discharge machining gap so that the degree of surface roughness of a surface on which the electric discharge machining is performed is inhibited from becoming higher. In addition, when the reversed polarity machining process is to be finished, the electric discharge is cut off without fail. Furthermore, during the reversed polarity machining process, a large machining voltage is applied to the electric discharge machining gap so that the electric discharges occur in a stable manner, while it is possible to perform the machining process with a low degree of surface roughness.    Patent Document 1: Japanese Patent Application Laid-open No. H11-347844